I don't does group study at all. It gives losses more than the benefits like deviating from studies by gossips and unwanted lengthy discussions. There might be benefit of clearing our doubts from our friends. But I prefer studying alone and I have various other methods to clear my doubts.
I do group studies with some of my friend. In a group all the people spearhead towards a single aim. We decide a particular topic and share our views and knowledge. Group study is helpful for fast revision and discuss deeply.
I cannot do group study now since I don't have any friends nearby but I used to group study during my college days. We used to study in sub-groups of 2-3 person each where each will study different portions. Later on, all of us sit together and explain the topics to others. This way we could cover huge volumes of portion is small time. There is a single disadvantage I feel over group study. When one of the members is not interested, he will ruin the study since he will be a source of distraction.